1. Field of the Invention
The present invention generally relates to an air-fuel ratio control apparatus, and more particularly to an apparatus for controlling an air-fuel ratio of an air-fuel mixture to an internal combustion engine having an evaporated fuel purge system.
2. Description of the Related Art
Recently, there have been many automotive engines provided with an evaporated fuel purge system. In the evaporated fuel purge system, evaporated fuel from a fuel tank is supplied to a canister via a vapor line between the fuel tank and the canister. The canister contains absorbent such as active carbon, and the evaporated fuel from the fuel tank is absorbed by the absorbent of the canister. The canister is connected to an intake passage of the engine through a purge line. The evaporated fuel purge system supplies or discharges the absorbed fuel of the canister to the intake passage through a purge control valve, arranged in the purge line, under a prescribed operating condition of the engine.
Generally, higher combustion efficiencies of automotive engines with desired exhaust emission can be acquired if an air-fuel ratio of an air-fuel mixture, supplied to the engine, is set to approximately the stoichiometric air-fuel ratio. Thus, for automotive engines vehicles requiring higher combustion efficiencies, it is necessary to perform an air-fuel ratio feedback control procedure in which the air-fuel ratio is determined in accordance with the stoichiometric air-fuel ratio.
U.S. patent application Ser. No. 08/085,571 filed on Jun. 30, 1993, assigned to the assignee of the present invention, discloses a proposed air-fuel ratio control apparatus for an internal combustion engine having an evaporated fuel purge system.
In the proposed air-fuel ratio control apparatus, the air-fuel ratio controlling is performed based on the principles that the purge ratio, indicating the ratio of a purge fuel flow rate to an intake air flow rate, is constant, and that the air-fuel mixture is not affected by the purge fuel flow. Based on the principles, the purge control valve is actuated with a drive signal in accordance with the constant purge ratio. Based on the principles, a correction factor is determined in accordance with the purge ratio, and a corrected fuel injection time is determined by multiplying a basic fuel injection time by the correction factor. A fuel injection valve, supplying fuel to the engine, is actuated in accordance with the corrected fuel injection time.
However, the purge control valve has a control delay in response to a drive signal. When the engine is operating in a transient condition in which the intake air flow rate abruptly changes, the purge ratio cannot be maintained at a constant level on the proposed air-fuel ratio control apparatus. The air-fuel ratio of the air-fuel mixture, supplied to the engine at this time, is considerably affected by the purge fuel flow due to the control delay of the purge control valve. The air-fuel ratio considerably deviates from the stoichiometric air-fuel ratio, causing the exhaust emission to deteriorate.